Chamber configuration for confining a plasma

ABSTRACT

A plasma confining assembly for minimizing unwanted plasma formations in regions outside of a process region in a process chamber is disclosed. The plasma confining assembly includes a first confining element and second confining element positioned proximate the periphery of the process region. The second confining element is spaced apart from the first confining element. The first confining element includes an exposed conductive surface that is electrically grounded and the second confining element includes an exposed insulating surface, which is configured for covering a conductive portion that is electrically grounded. The first confining element and the second confining element substantially reduce the effects of plasma forming components that pass therebetween. Additionally, the plasma confining assembly may include a third confining element, which is formed from an insulating material and disposed between the first confining element and the second confining element, and proximate the periphery of the process region. The third confining element further reduces the effects of plasma forming components that pass between the first confining element and the second confining element.

BACKGROUND OF THE INVENTION

The present invention relates to apparatus and methods for processingsubstrates such as semiconductor substrates for use in IC fabrication orglass panels for use in flat panel display applications. Moreparticularly, the present invention relates to improved techniques forcontrolling plasma formation in a process chamber of a plasma reactor.

The use of plasma-enhanced processes in the manufacture ofsemiconductor-based products (such as integrated circuits or flat paneldisplays) is well known. In general, plasma-enhanced processes involveprocessing a substrate in a process chamber of a plasma reactor. In mostplasma reactors, a plasma may be ignited and sustained by supplying agas containing appropriate etchant or deposition source gases into theprocess chamber and applying energy to those source gases torespectively etch or deposit a layer of material on the surface of thesubstrate. By way of example, capacitive plasma reactors have beenwidely used to process semiconductor substrates and display panels. Incapacitive plasma reactors, a capacitive discharge is formed between apair of parallel electrodes when RF power is applied to one or both ofthe electrodes.

Although the plasma predominantly stays in the process area between thepair of electrodes, portions of the plasma may fill the entire chamber.A plasma typically goes where it can be sustained, which is almostanywhere in the chamber. By way of example, the plasma may fill theareas outside the process region such as the bellows of the pumpingarrangement. If the plasma reaches these areas, etch, deposition and/orcorrosion of the areas may ensue, which may lead to particlecontamination inside the process chamber, and/or which may reduce thelifetime of the chamber or chamber parts. Furthermore, a non-confinedplasma may form a non uniform plasma, which may lead to variations inprocess performance.

Accordingly, there are continuing efforts to produce plasmas, which areconfined to the process region, and thus more stable. Confined plasmastend to ensure efficient coupling of energy to discharges, enhanceplasma uniformity, and increase plasma density, all of which lead tobetter processing uniformity and high yields on processed substrates.There are various ways to achieve a confined plasma. One approach usesexternal magnetic fields to confine the plasma. Another approach uses aconfinement ring to confine the plasma. The confinement ring istypically formed from an insulating material that physically blocks theplasma pumping passage, thereby confining the plasma. Both approacheshave proven to be highly suitable for plasma processing, and moreparticularly for improving process control and ensuring repeatability.Although these approaches work well, there are continuing efforts toimprove plasma confinement, and more particularly for minimizing and/oreliminating the unwanted plasma formation in the region outside of theprocess region of the process chamber. For example, depending on power,pressure and chemistries, a relatively strong electric field and asubstantial amount of residue ionic species may be present outside theconfined plasma region, which can further induce glowing dischargeoutside the confined process region, i.e., plasma un-confinement.

SUMMARY OF THE INVENTION

The invention relates, in one embodiment, to a plasma confining assemblyfor minimizing unwanted plasma formations in regions outside of aprocess region in a process chamber. The assembly includes a firstconfining element positioned proximate the periphery of the processregion. The first confining element includes an exposed conductivesurface that is electrically grounded. The assembly further includes asecond confining element positioned proximate the periphery of theprocess region and spaced apart from the first confining element. Thesecond confining element includes an exposed insulating surface, whichis configured for covering a conductive portion that is electricallygrounded. The first confining element and the second confining elementare arranged to substantially reduce the effects of plasma formingcomponents (e.g., charged particles and/or electric fields) that passtherebetween.

In other embodiments, the plasma confining assembly additionallyincludes a third confining element formed from an insulating material.The third confining element is disposed between the first confiningelement and the second confining element and proximate the periphery ofthe process region. The third confinement element is arranged tophysically contain a plasma inside the process region and tosubstantially reduce the effects of plasma forming components that passbetween the first confining element and the second confining element.

The invention relates, in another embodiment, to a plasma reactor forprocessing a substrate. The plasma reactor includes a chamber havingchamber walls. The plasma reactor further includes an electrodearrangement configured for generating an electric field, which issufficiently strong to both ignite and sustain a plasma for theprocessing within the chamber. The electrode arrangement includes afirst electrode and a second electrode, which are spaced apart therebydefining a process region therebetween. The plasma reactor additionallyincludes a plasma confinement assembly for preventing the plasma fromforming outside of the process region. The plasma confinement assemblyincludes a first ring, which is configured to surround the firstelectrode, and a second ring, which is configured to surround the secondelectrode. The first ring includes an exposed conductive surface that iselectrically grounded, and the second ring includes an exposedinsulating surface covering a non exposed conductive element that isgrounded. The plasma confinement assembly is arranged to substantiallyreduce the effects of plasma forming components that pass between thefirst confinement ring and the second confinement ring.

In some embodiments, the plasma confining assembly also includes apressure control ring formed from a dielectric medium and disposedbetween the first and second rings. The pressure control ring isconfigured for physically confining a plasma within the process region,while permitting the passage of process gases to pass therethrough.

In other embodiments, the first ring includes an inner ring and an outerring. The inner ring is formed from a dielectric medium and disposedbetween the first electrode and the outer ring, and the outer ringincludes a grounded conductive surface. In other embodiments, the secondring includes an inner ring and an outer ring. The inner ring is formedfrom a dielectric medium and disposed between the second electrode andthe outer ring, and the outer ring includes a conductive core, which iscovered by the insulating layer, and which is electrically grounded.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 illustrates a plasma reactor, in accordance with one embodimentof the present invention.

FIG. 2 shows a broken away view of the plasma reactor of FIG. 1, inaccordance with one embodiment of the present invention.

FIG. 3 illustrates a lower ring, in accordance with one embodiment ofthe present invention.

FIG. 4 illustrates an upper ring, in accordance with one embodiment ofthe present invention.

FIG. 5 illustrates an upper ring, in accordance with one embodiment ofthe present invention.

FIG. 6 illustrates a plasma reactor, in accordance with one embodimentof the present invention.

FIG. 7 shows a conductive upper ring and a conductive lower ring.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in detail with reference toa few preferred embodiments thereof as illustrated in the accompanyingdrawings. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one skilled in the art, thatthe present invention may be practiced without some or all of thesespecific details. In other instances, well known process steps have notbeen described in detail in order not to unnecessarily obscure thepresent invention.

It has been discovered that unwanted discharges or plasmas may beencountered when large charged particle fluxes and/or large electricfields are present in regions outside of the process region of theprocess chamber. As the term is employed herein, the process regionrefers to the region of the process chamber used for processing asubstrate, for example, the area directly above the substrate. Withregards to charged particles, charged particles leaving the processregion may collide with the wall of the process chamber, and as a resultgenerate secondary electrons that can ignite and/or sustain a plasma.With regards to the electric field, an electric field can accelerate theelectrons causing them to collide with the gas molecules of the processgas, which as a result can ionize and initiate a plasma. In addition,charged particles tend to follow the electric field lines and thus strayelectric field lines may guide more charged particles into regionsoutside of the process region of the process chamber. For example,electric fields can accelerate charged particles in a direction towardsthe walls of the process chamber. This acceleration and subsequentcollision between the charged particles and the chamber walls maygenerate secondary electrons, which may ignite and/or sustain a plasma.

The invention therefore pertains to an improved method and apparatus forconfining a plasma to a process region of a process chamber. Moreparticularly, the invention pertains to a system for minimizing and/oreliminating unwanted discharges (or plasma formations) in regionsoutside of the process region of the process chamber. For ease ofdiscussion, the regions outside of the process region will herein bereferred to as the outer regions of the process chamber. One aspect ofthe invention relates to reducing the density of charged particles inthe outer regions of the process chamber. For example, by absorbingparticles on surfaces before they reach the outer regions or by notallowing the particles to pass to the outer regions. Another aspect ofthe invention relates to attenuating the electric field (used to formthe plasma) in the outer regions of the process chamber. For example, bydirecting electric fields away from the outer regions. The invention isparticularly useful in plasma processing systems that utilize capacitivedischarges to form the plasma.

In accordance with one embodiment of the present invention, there isprovided a plasma confining assembly for confining a plasma in a processregion of a process chamber. The assembly includes a first confiningelement, which is positioned towards a side of the process region, andwhich includes an exposed conductive surface that is electricallygrounded. The assembly also includes a second confining element, whichis positioned towards the side of the process region and spaced apartfrom the first confining element, and which includes an exposedinsulating surface covering an un-exposed conductive portion that iselectrically grounded. In some configurations, the plasma confiningassembly may include an insulated pressure control ring, which is alsopositioned towards the side of the process region, and between the firstand second confining elements.

Although the above assembly is not limited to the following, it isgenerally believed that the above assembly achieves plasma confinementby capturing charged particles streaming out of the process region ofthe process chamber and shielding portions of the electric fieldstraying outside of the process region of the process chamber. Forinstance, the assembly is configured to direct charge particles to theconductive surface of the first confining element and sink particlestherethrough to ground so as to reduce the density of charged particlesin regions outside of the process region. The assembly is alsoconfigured to neutralize some of the charged particles on the pressurecontrol ring so as to reduce the density of charged particles in regionsoutside of the process region. Moreover, the assembly is configured toredirect stray electric fields through the conductive surface and toground so as to reduce electric fields in regions outside of the processregion. For example, the direction of electric fields can be altered sothat the electric fields no longer have a line of sight that extends tothe outer regions of the process chamber or the chamber walls.

Embodiments of the invention are discussed below with reference to FIGS.1-7. However, those skilled in the art will readily appreciate that thedetailed description given herein with respect to these figures is forexplanatory purposes as the invention extends beyond these limitedembodiments.

FIG. 1 is a schematic diagram of a plasma reactor 10, in accordance withone embodiment of the present invention. Plasma reactor 10 includes aprocess chamber 12, a portion of which is defined by a top plate 13 andchamber walls 14, and within which a plasma 17 is both ignited andsustained for processing a substrate 18. Substrate 18 represents thework piece to be processed, which may represent, for example, asemiconductor substrate to be etched or otherwise processed or a glasspanel to be processed into a flat panel display. In most embodiments,the process chamber 12 is arranged to be substantially cylindrical inshape, and the chamber walls are arranged to be substantially vertical.Furthermore, the top plate 13 and chamber walls 14 are generallygrounded and formed from a suitable material such as aluminum. In theembodiment shown, the top plate 13 and chamber walls 14 are electricallyconnected (at interface 19) and grounded as indicated at 16. The topplate 13 and chamber walls 14 may also include insulating surfaces 15,which may be formed from a suitable dielectric material or in the caseof an aluminum top plate or chamber wall the insulating surface mayformed from anodized aluminum. It should be understood, however, thatthe above mentioned configurations are not a limitation and that theymay vary according to the specific design of each plasma reactor. Forexample, the chamber walls may be sloped or the top plate may beseparately grounded.

In most embodiments, the substrate 18 is introduced into the processchamber 12 and disposed between a pair of parallel electrodes, and moreparticularly a bottom electrode 20 and a top electrode 24. The bottomand top electrodes 20, 24 define a process region 26 therebetween. Asshown, the plasma 17 is confined vertically between the bottom electrode20 and the top electrode 24. The bottom electrode 20 may serve as achuck for supporting and holding the substrate 18 during processing. Byway of example, the chuck may be an electrostatic chuck, a vacuum chuck,or a mechanical chuck. The electrodes 20, 24 are also generally arrangedto be substantially cylindrical in shape and axially aligned with anaxis 11 of the process chamber 12 such that the process chamber 12 andthe electrodes 20, 24 are cylindrically symmetric. In addition, theelectrodes 20, 24 are generally configured with similar dimensions, forexample, the electrodes may have the same diameter. It should beappreciated, however, that the size, shape and placement of theelectrodes may vary according to the specific design of each plasmareactor. Furthermore, an edge ring 27 may be provided to improve theelectrical and mechanical properties of processing near the substrate'sedge, as well as to shield the bottom electrode 20 from reactants (i.e.,ion bombardment). The edge ring 27 is arranged to surround the edge ofthe substrate 18 and cover the bottom electrode 20. The edge ring 27 isgenerally formed from a suitable dielectric material such as ceramic,quartz, plastic and the like.

In general, the electrodes 20, 24 are configured to deliver highfrequency energy into the process region 26 of the process chamber 12 soas to ignite and sustain the plasma 17. More specifically, and in theembodiment shown, the top electrode 24 is coupled to the top plate 13,which is grounded as indicated at 16, and the bottom electrode 20 iscoupled to an RF power supply 28 via a matching network 30. Matchingnetworks are generally well known in the art and for the sake of brevitywill not be discussed in great detail herein. The top electrode 24 isarranged to be electrically continuous with the top plate 13, andtherefore the top electrode 24 is also grounded (e.g., completes RFcircuit). The top electrode 20 may be formed form a suitable conductivematerial such as silicon. Furthermore, the RF power supply 28 isconfigured to supply the bottom electrode 20 with RF energy. In theillustrated embodiment, the RF power supply 28 consists of a first RFpower supply 28A and a second RF power supply 28B. The first RF powersupply is configured to apply a first RF frequency to the bottomelectrode 20 and the second RF power supply is configured to apply asecond RF frequency to the bottom electrode 20. By way of example, thefirst RF frequency may be about 27 MHz and the second RF frequency maybe about 2 MHz. As is generally well known, a higher frequency is usedto control the density of the plasma and the quantity of ions incidenton the substrate, while a lower frequency is used to control the energyof the ions incident on the substrate. Moreover, the bottom electrode 24may be formed from a suitable conductive material such as aluminum.

In the plasma reactor shown in FIG. 1, the RF frequencies are driven onthe bottom electrode 20 and capacitively coupled to the plasma 17. Theplasma 17 and its sheaths oscillate periodically at these radiofrequencies and the positive ions in the plasma 17 are accelerated bythe sheath voltage to the grounded top electrode 24. As should beappreciated, the driving RF currents supplied to the bottom electrode 20should be balanced with the plasma ion current on the top electrode 24plus any ion flux leaking out of the process region 26. In the sense ofan electric circuit, this is continuity for the RF current. In an idealsituation, the RF current comes in the bottom electrode 20 passesthrough the plasma 17 to the top electrode 24, and then travels throughthe top plate 13 to the chamber 14, where it finally terminates to theground 16 and completes a round trip.

In accordance with another aspect of the invention, therefore, goodelectrical conductance through the desired RF return path is provided toenhance plasma confinement. A good electrical path helps to drive the RFfrequencies and therefore to efficiently couple RF to the plasma. Morespecifically, a good electrical path tends to attract more ion flux tothe top electrode which as a result reduces the ion flux to the otherundesirable paths such as outside of the confined region or processregion 26. Accordingly, the plasma is better confined.

In one embodiment, the top electrode 24 is formed from a low resistivitymaterial so as to improve the electrical conductance through the RFreturn path. By way of example, the top electrode may be formed from asuitable semiconductor (e.g., silicon) having a resistivity (bulk) inthe range of about 10 ohm-cm to about 0.01 ohm-cm.

In another embodiment, good electrical contact is made between the topplate 13 and the chamber walls 14 (interface 19) so as to improve theelectrical conductance through the RF return path. In this embodiment,the good electrical contact is made by introducing a RF gasket 21 at theinterface 19, i.e., between the top plate 13 and the chamber wall 14.The RF gasket 21 is configured to be a compliant electrical conductor.By compliant it is meant that the RF gasket 21 takes the shape of thetop plate 13 and the chamber wall 14 at the interface 19. In oneconfiguration, the cross sectional shape of the RF gasket 21 takes theform of a spiral (similar to a thin circular spring). The spiral allowsthe RF gasket 21 to become compressed between the top plate 13 and thechamber wall 14 so as to couple the two together. When compressed, eachspiral contacts an adjacent surface making the RF gasket 21 anintegrated conductive element that provides an electrical path betweenthe top plate 13 and the chamber walls 14. The RF gasket is generallyformed from a suitable conductive material such as stainless steel.

Although the bottom electrode is shown and described as being coupled toa pair of RF power supplies, it should be understood that otherconfigurations may be used to accommodate different process chambers orto conform to other external factors necessary to allow the coupling ofenergy. For example, a single frequency plasma reactor or a dualfrequency plasma reactor with one RF power supply coupled to the bottomelectrode and another coupled to the top electrode may be used.

In addition, a gas injector (not shown) is typically provided forreleasing a single gaseous source material or a mixture of gaseoussource materials into the process chamber 12, and more particularly theprocess region 26 between the top and bottom electrodes 20, 24. By wayof example, a gas injection port(s) may be built into the walls of theprocess chamber itself or through a shower head arrangement in the topelectrode. An exhaust port 34 is also provided for exhausting spentgases formed during processing. As shown, the exhaust port 34 is locatedin an outer region 36 of the process chamber 12 and disposed between thechamber walls 14 and the bottom electrode 20. The exhaust port 34 isgenerally coupled to a turbomolecular pump (not shown), which is locatedoutside of the process chamber 12, and which is arranged to maintain theappropriate pressure inside the process chamber 12. Furthermore,although the exhaust port is shown disposed between the chamber wallsand the bottom electrode, the actual placement of the exhaust port mayvary according to the specific design of each plasma processing system.For example, the exhausting of gases may also be accomplished from portsbuilt into the chamber walls. Gas systems that include gas injectors andexhaust ports are well known in the art and for the sake of brevity willnot be discussed in anymore detail herein.

In order to create the plasma 17, a process gas is typically input intothe process region 26 from the gas injection port or ports (not shown).Power is then supplied to the bottom electrode 20 using the RF powersource 28, and a large electric field (shown graphically with electricfield lines 95) is produced inside the process chamber 12. Most of theelectric field lines are contained between the bottom electrode 20 andtop electrode 22, although some electric field lines may stray beyondthis boundary. The electric field accelerates the small number ofelectrons present inside the process chamber 12 causing them to collidewith the gas molecules of the process gas. These collisions result inionization and initiation of the plasma 17. As is well known in the art,the neutral gas molecules of the process gas when subjected to thesestrong electric fields lose electrons, and leave behind positivelycharged ions.

As a result, positively charged ions, negatively charged electrons andneutral gas molecules are contained inside process chamber 12. Duringprocessing, the ions are typically made to accelerate towards thesubstrate where they, in combination with neutral species, activatesubstrate processing, i.e., etching, deposition and/or the like.

Most of the charged species (e.g., ions and electrons) are containedinside the process region 26 to facilitate substrate processing,although some charged species may stray outside of this region (forexample through the pumping passage 37).

In accordance with one embodiment of the invention, during operation ofthe plasma reactor 10, the plasma discharge generated between theelectrodes 20 and 24 is substantially confined to the process region 26by providing a confinement system 50.

For ease of discussion, FIG. 2 depicts a broken away side view of theplasma reactor 10 in accordance with this embodiment so as to provide acloser look at the confinement system 50 and its functions. Theconfinement system 50 includes a pressure control ring 52 (orconfinement ring) and a confining assembly 51, including an upper ring53 and a lower ring 54. In the embodiment shown, the confining system 50is disposed in a pumping passage 37, which is positioned to the side ofthe process region 26. The confinement system 50 is configured forconfining the plasma 17 to the process region 26 of the process chamber12 and for minimizing and/or eliminating unwanted plasma formations inthe outer regions 36 of the process chamber 12. In most cases, thepressure control ring 52 is configured for physically confining theplasma 17 and for neutralizing a portion of the charged particles thatstray out of the process region 26. In addition, the confining assembly51 is configured for capturing (neutralizing) a portion of the chargedparticles that stray out of the process region 26 and for attenuatingthe electric field lines that stray out of the process region 26.Moreover, the confining assembly 51 may be configured to at least inpart physically confine the plasma 17.

The pressure control ring 52 is an annular ring arranged to surround theprocess region 26 and to control the gas pressure in the plasma 17. Thepressure control ring 52 is generally disposed between the planes thatdefine the surfaces of the electrodes 20 and 24, and more particularlyis disposed within the pumping passage 37 adjacent to the process region26. As shown, the pressure control ring 52 physically blocks a portionof the pumping passage 37 and thus it can restrict the plasma 17 fromradially leaving the process region 26 (e.g., confinement). Morespecifically, the pressure control ring 52 has an inner surface 56,which is exposed to the process region 26 and an outer surface 58, whichis exposed to the outer region 36. The pressure control ring 52 also hasan upper surface 57, which is spaced apart from the upper ring 54, and alower surface 59, which is spaced apart from the lower ring 53. As such,the pressure control ring 52 is disposed between the upper and lowerrings 53, 54. In addition, the pressure control ring 52 is generallyformed from an insulating material and can be either a single ring orseveral rings.

Furthermore, the pressure control ring 52 generally includes a pluralityof passages 60 extending from the inner surface 56 to the outer surface58. The passages 60 are dimensioned to permit by-product gases or spentgases (formed during processing) to pass through while substantiallyconfining the plasma to the process region 26. In addition, the passages60 are configured to substantially neutralize charged particles (createdin the plasma) that stream out of the process region 26. Morespecifically, the passages are appropriately proportioned such that thedistance a charged particle must travel in the passages is substantiallylonger than the mean free path of the charged particle so that mostexiting particles make at least one collision with the walls of thepassage. These collisions tend to neutralize the charge of the particle.Accordingly, the tendency for a discharge outside the process region isreduced.

As should be appreciated, the width (from 57 to 59) and thickness (from56 to 58) of the pressure control ring 52 can be adjusted to enhanceplasma confinement. For example, a pressure control ring with arelatively large width and thickness generally lead to better plasmaconfinement since a large width or a large thickness restricts gaspumping conductance and increases the residence time for charged speciesand radicals to pass through, thereby increasing the chance of chargeexchange and neutralization. In particular, a wider pressure controlring provides more surface area for charged species to be exchanged andneutralized and for radicals to be quenched, while a pressure controlring with relatively large thickness further reduces the direct line ofsite gap from the confined plasma region to the outside, therebyreducing the chance of the direct electrical breakdown.

In one configuration, the pressure control ring includes a stack ofannular rings formed from a suitable dielectric material such as highquality fused silica or quartz. When assembled, the annular rings areseparated by spacers (not shown) that also may be formed from a suitabledielectric material such as quartz. The spacers may be washers or raisedregions of the annular rings. Screws may be threaded through the ringsand washers to form a rigid structure. In addition, the pressure controlring can be supported directly or indirectly by connecting it to someportion of the process chamber, for example to the upper ring. Thespaces between adjacent rings form distinct parallel circumferentialpassages or slots. The slots extend essentially around the fullcircumference of the pressure control ring, being interrupted only bythe spacers. An example of a pressure control ring such as the onedescribed above may be found in commonly assigned U.S. Pat. No.5,534,751 to Lenz, et al, which is herein incorporated by reference. Inaddition, the pressure control ring may be arranged to move between afirst and second position in order to effect the pressure at the surfaceof the substrate. This may further enhance the processing performance ofthe plasma reactor. An example of a moving pressure control ring such asthis may be found in commonly assigned U.S. Pat. No. 6,019,060 to Lenz,which is also incorporated herein by reference.

With regards to the lower ring 53, the lower ring 53 is an annular ringarranged to concentrically surround the bottom electrode 20. In theembodiment shown, the lower ring 53 is disposed between the bottomelectrode 20 and the chamber wall 14, and more particularly adjacent theside of the bottom electrode 20 so as to provide space for the exhaustport 34. It should be noted, however, that in some instances the lowerring may provide secondary confinement (e.g., physical) by extendinginto the exhaust port, and may include holes or passages for allowingthe gases to pass therethrough. In general, the top surface 70 of thelower ring 53 defines a lower passage of the pumping passage 37. Asshown, a top surface 70 of the lower ring 53 is at about the same levelas the plane that defines the top surface of the bottom electrode 20.Furthermore, the top surface 70 and a side surface 72 of the lower ring53 are exposed to the interior of the process chamber 12.

The lower ring 53 includes an inner side ring 76 and an outer side ring78. The inner side ring 76, as its name suggests, is located in an innerportion of the lower ring 53, and thus it is adjacent to the bottomelectrode 20. The outer side ring 78, on the other hand, is located inan outer portion of the lower ring 53. In general, the outer side ring78 is configured with a conductive core 80, which is electricallygrounded as indicated at 84 (providing an RF return path), and which iseither fully or partially surrounded by an insulating surface 82. Theinsulating surface 82 is arranged to cover at least the portions of theouter side ring 78 that are exposed to the process chamber 12, forexample the top surface 70 and the side surface 72. Although not shown,the insulating surface may also cover the unexposed portions of theouter side ring 78. Furthermore, the inner side ring 76 is formed froman electrically insulating material so as to isolate the outer side ring78 from the RF-driven bottom electrode 20 and to prevent any electricalbreakdown and arcing therebetween. By way of example, the inner ring 76may be formed from dielectric, quartz, ceramic, plastic and may have airpockets arranged therein.

In one embodiment, the outer side ring is formed from aluminum, and theexposed surfaces, for example, top surface 70 and side surface 72 areanodized aluminum surfaces. As should be appreciated, the anodizedsurfaces are insulating surfaces, and thus the ambient RF iscapacitively terminated to the inside aluminum (conductive core) throughthe anodized layer. The capacitance is typically low because of the thinanodized surface. This configuration of the outer side ring is helpfulfor plasma confinement, and will be discussed in greater detail below.

With regards to the upper ring 54, the upper ring 54 is an annular ringarranged to concentrically surround the upper electrode 24, and isgenerally attached to the top plate 13 of the process chamber 12. In theembodiment shown, the upper ring 54 is disposed between upper electrode20 and the chamber wall 14, and more particularly proximate the side ofthe upper electrode 24. The upper ring 54 includes an upper surface 86,a bottom surface 88, an inside surface 90 and an outside surface 92. Insome embodiments, the side surface 90 of the upper ring 54 may beadjacent to the top electrode 24, and in other embodiments (as shown), aspace may be provided between the top electrode 24 and the insidesurface 90 of the upper ring so as to provide clearance for thermalexpansion. As shown, the outside surface 92 of the upper ring 54 isconfigured to extend farther away from the axis 11 of the processchamber 12 than the side surface 72 of the lower ring 53. In general,the bottom surface 88 defines an upper passage of the pumping passage37. Furthermore, the bottom surface 88 is at about the same level as theplane that defines the top surface of the upper electrode 24. It shouldbe noted, however, that this is not a limitation and that the width ofthe upper ring (from surface 86 to surface 88) may be altered torestrict gas pumping in a manner similar to the pressure control ring soas to enhance plasma confinement. By way of example, the bottom surface88 may extend in a direction towards the lower ring 53. As shown, thebottom surface 88 and the outside surface 92 of the upper ring 54 areexposed to the interior of the process chamber 12. Furthermore, thebottom surface 88 of the upper ring 54 is in a position that faces thetop surface 70 of the lower ring 53. In most embodiments, the bottomsurface and the top surface are substantially parallel to one anotherand perpendicular to the axis 11.

The upper ring 54 is formed from a suitable conductive material and iselectrically grounded. The upper ring 54 can be indirectly groundedthrough the top plate 13 or directly grounded as indicated by 94 in FIG.1 (providing both a DC path and a RF return path). Furthermore, theupper ring 54 is arranged to be substantially resistant to etching bythe plasma 17 or to contribute substantially no metal contamination. Byway of example, the upper ring 54 can be formed from a bare metal, SiC,Si sputtered over metal or the like.

The confining assembly 51, including the lower ring 53 and the upperring 54, has several functions that can have large effects on plasmaconfinement. One function includes substantially neutralizing chargedparticles that stream out of the process region. This is accomplished atleast in part by sinking charged species at the upper ring 54. Morespecifically, the grounded bottom surface 88 acts as a charge sink ordrain for positive ions to get neutralized before escaping the confinedprocess region. While not wishing to be bound by theory, it is generallybelieved that the RF voltage creates a DC potential between theconductive and insulated surfaces during processing. The DC potentialguides charged species to either the top insulated surface 70 of thelower ring 53 or to the bottom conductive surface 88 of the upper ring54. As a result, the exiting particles make at least one collision withthe top surface 70 or the bottom surface 88. Upon collision, a currentflow is created that essentially removes the charge (via ground) fromthe charged particle, which as a result tends to neutralize the chargeof the particle. For example, the conductive bottom surface 88 providesa DC ground path for charged species. As such, the density of chargedparticles in the outer region are substantially reduced. Accordingly,the tendency for a discharge outside the process region is reduced.

In one embodiment, the surface area of the grounded and conductivebottom surface 88 is used to control the amount of sinking particles. Ineffect, the greater the surface area of the bottom surface, the greaterthe effect in sinking charged particles.

Another function includes shielding the ambient RF (or stray electricfield lines) to reduce the strength of the electric field outside theconfined process region 26. This is accomplished by attracting theelectromagnetic fields of the driving RF that would otherwise divergeradially outward to outside the confined process region 26 to theconductive elements of the upper and lower rings, i.e., the conductivecore and conductive bottom surface. As shown in FIG. 2, a portion of thediverging or stray electric fields (shown graphically with electricfield lines 96) are removed from the outer region 36 via the outer sidering 78 of the lower ring 53, as well via the upper ring 54. For ease ofdiscussion, the electric field lines 96, which are removed by the outerside ring 78 are designated 96A, and the electric field lines, which areremoved by the upper ring 54 are designated 96B. The electrical fieldlines 96B, which are incident on the upper ring, tend to beperpendicular because of the exposed conductive surface, while theelectrical field lines 96A, which are incident on the outer side ring,tend to be angled because of the insulated top surface. The strayelectric field lines 96A couple through the insulated top surface 70 tothe conductive core 80, and travel through the conductive core 80 toground 84. In addition, the stray electric field lines 96B travel to theconductive bottom surface 88 and through the conductive bottom surface88 to ground 94. As should be appreciated, both the conductive bottomsurface and the insulated conductive core provide an RF return path. Assuch, the stray electric fields in the outer region are substantiallyreduced by both the upper ring and the outer side ring of the lowerring. Accordingly, the tendency for a discharge outside the processregion is reduced.

Furthermore, charged particles tend to follow electric field lines 96A&B and therefore by reducing the electric fields in the outer region thedensity of charged particles in this region is also reduced. Moreover,the charged particles tend to be directed to the conductive bottomsurface of the upper ring 54 by electric field 96B, which as a result,neutralizes the charged particles, especially the ionic species. Thecharged particles also tend to be directed to the surfaces of thepressure control ring 52, which as a result, neutralizes some of thecharged particles. Accordingly, the tendency for a discharge outside theprocess region is reduced.

In one embodiment, the surface area of the bottom surface 88, as well asthe surface area of the top surface 70 is used to control the amount ofshielded field lines. In effect, the greater the surface area, thegreater the effect in shielding electric fields.

In alternate embodiment, and as shown in FIG. 3, the lower ring 53 mayinclude a first ring 112, a second ring 114, a third ring 116 and a topring 118. In this figure, the rings are produced in accordance with theteachings of the invention set forth above with regard to inner sidering 76 and the outer side ring 78. That is, the first ring 112corresponds to the inner side ring 76, the second ring 114 correspondsto the conductive core 80, the third ring 116 corresponds to the sidesurface 72, and the top ring 118 corresponds to the top surface 70. Assuch, the first ring 112, the third ring 116 and the top ring 118 areformed from a suitable insulating material, while the second ring 114 isformed from a suitable conductive material. The second ring 114 is alsoelectrically grounded as indicated by 84. In another embodiment, thefirst ring 112, the third ring 116 and the top ring 118 may represent anintegrally formed structure such that the second ring is embeddedtherein. In another embodiment, the first ring 112, the third ring 116and the top ring 118 may represent a composite structure, where each ofthe rings is formed from a different dielectric material. In yet anotherembodiment, the top ring may be an extension of the edge ring 27.

In one configuration, the lower ring includes an annular dielectric bodyhaving a first portion and a second portion, which encircle the bottomelectrode, and a conductive core, which comprises a tube-shaped portionand an inwardly-protruding portion. The tube shape portion substantiallysurrounds and shields the lower portion of the annular dielectric body,while the inwardly-protruding portion is embedded within the annulardielectric body itself. An example of a lower ring such as the onedescribed above may be found in commonly assigned U.S. Pat. No.5,998,932 to Lenz, which is herein incorporated by reference.

In another alternate embodiment, and as shown in FIG. 4 the upper ring54 can be split into two rings, an inner upper ring 100 and an outerupper ring 102. The inner upper ring 100, as its name suggests, islocated in an inner portion of the upper ring 54, and thus it isproximate to the upper electrode (not shown in this figure). The outerupper ring 102, on the other hand, is located in an outer portion of theupper ring 54. The inner upper ring 100 is formed from a suitabledielectric material, while the outer upper ring 102 is formed from asuitable conductive material and electrically grounded as indicated at94. The inner upper ring 100 is configured to reduce particle generationdue to plasma sputtering on the inner edge of the upper ring 54 that isdirectly exposed to the confined plasma (plasma 17 as shown in FIGS. 1 &2). The outer upper ring 102, however, is produced in accordance withthe teachings of the invention set forth above with regards to thesingle piece upper electrode. That is, the residue ions still getneutralized on the conductive surface of the grounded outer upper ring102 before escaping to outside the confined process region, and theelectric field still gets shielded by the grounded conductive outerupper ring 102.

In another alternate embodiment, and as shown in FIG. 5 the upper ring54 can include a top layer 106 and a bottom layer 108. The top layer 106is disposed in an upper portion of the upper ring 54, and thus it isproximate to the top plate (not shown in this figure). The bottom layer108 is disposed in a bottom portion of the upper ring 54, and thus it isexposed to the interior of the process chamber 12. The top layer may beformed from any suitable material whether conductive or insulating. Onthe other hand, the bottom layer 108, which is electrically grounded asindicated by 94, is formed from a suitable conductive material. In thisembodiment, the bottom layer 108 is produced in accordance with theteachings of the invention set forth above with regards to the singlepiece upper electrode. That is, the residue ions still get neutralizedon the conductive surface of the grounded bottom layer 108 beforeescaping to outside the confined process region, and the electric fieldstill gets shielded by the grounded bottom layer 108. In oneimplementation, the bottom layer 108 may be sputtered onto the top layer106. Because there is not much current, the sputtered layer can be avery thin layer of conductive material.

In accordance with another embodiment of the present invention, theconfigurations of the upper and lower ring can be reversed. That is, theupper ring can have an insulating bottom surface, and the lower ring canhave a conductive top surface. The features of this embodiment may bebetter understood with reference to the figure that follows. FIG. 6illustrates a relevant portion of the plasma reactor 10 of FIG. 1,including process chamber 12, top plate 13, chamber walls 14, bottomelectrode 20, upper electrode 24, and pressure control ring 52. FIG. 6also illustrates a confining assembly 200 including an upper ring 202and a lower ring 204 having an inner side ring 206 and an outer sidering 208. The inner side ring 206 may respectively correspond to theinner side ring 76 illustrated in FIG. 1. The outer side ring 208, onthe other hand, includes a top surface 210, which is exposed to theinterior of the process chamber 12. In this embodiment, the outer sidering 208, and more particularly, the top surface 210 of the outer sidering is formed from a suitable conductive material and is grounded asindicated by 84. By way of example, the outer side ring 208 or the topsurface may be formed from a bare metal, SiC, or Si sputtered overmetal. Additionally, the outer side ring 208 may include a side surface214, which is also exposed to the interior of the process chamber 12. Insome configurations, the side surface 214 is formed from a suitableconductive material, while in other configurations, the side surface 214is formed from a suitable insulating material. For ease of discussion,the size, shape and position of the outer side ring 208 may respectivelycorrespond to the outer side ring 78 illustrated in FIG. 1. With regardsto the upper ring 202, the upper ring 202 includes a bottom surface 216,which is also exposed to the interior of the process chamber 12. Asshown, the bottom surface 216 faces the top surface 210. In thisembodiment, the upper ring 202, and more particularly, the bottomsurface 216 is formed from a suitable insulating material. By way ofexample, the upper ring 202 may be formed from dielectric, ceramic,plastic, and the like. Like the outer side ring, the size, shape andposition of the upper ring 202 may respectively correspond to the upperside ring 54 illustrated in FIG. 1.

In a manner analogous to the confining assembly 51 of FIG. 1, theconfining assembly 200 of FIG. 6 can greatly improve plasma confinement.For example, since to the grounded top surface 210 is directly exposedto the interior of the process chamber 12, a clearly defined sheathforms on the top of this surface, and a voltage is built up across thesheath that can further guide positive ions to the surface. As such, thegrounded top surface acts as a charge sink or drain for positive ions toget neutralized before escaping the confined region. Accordingly, plasmaconfinement is substantially improved because positive ions that areleaking to the outside of the confined region are greatly reduced. Inaddition, the conductive top surface 210 effectively shields the ambientRF (or stray electric field lines) to reduce the strength of theelectric field outside the confined process region 26. Again, this isaccomplished by attracting the electromagnetic fields of the driving RFthat would otherwise diverge radially outward to outside the confinedprocess region 26 to the conductive elements of the lower ring, i.e.,the conductive top surface.

As discussed above, the combination of a conductive top surface (e.g.,outer side ring) and an insulating bottom surface (e.g., upper ring) orthe combination of a conductive bottom surface (e.g., upper ring) and aninsulating top surface (e.g., outer side ring) can greatly improveplasma confinement. Unfortunately, however, the combination of aconductive top surface (e.g., outer side ring) and a conductive bottomsurface (e.g., upper ring) can adversely effect plasma confinement. Tofacilitate discussion, FIG. 7 shows a confinement assembly 300 includinga lower ring 208 having a conductive top surface 210 and an upper ring54 having a conductive bottom surface 88. As shown, there is nearlyline-of-sight path between the outer edge of the upper ring 54 and theouter edge of the lower ring 208. The electrons or negative ions 302 maybecome trapped in the potential well defined by the sheaths formed onthe conductive bottom surface 88 of the upper ring 54 and the conductivetop surface 210 of the lower ring 208. Similar to the hollow cathodeeffect, these trapped negative species 302 oscillate back and forth inthe potential well. As a result, a glowing discharge can be inducedthrough the collisions of other ions and neutrals (not shown) with thetrapped negative species 302. Accordingly, either a combination of adielectric upper ring and an outer side ring with a conductive surfaceor a combination of a conductive upper ring and an outer side ring witha dielectric top surface is implemented to improve plasma confinement.

The above concepts have been extensively tested and proved to be validin experiments. A Langmuir probe and an E-field probe have beenimplemented in a dual frequency capacitive discharge reactor such as thecapacitively coupled Exelan™ plasma reactor, which is available from LamResearch Corporation of Fremont, Calif., to measure, respectively, theion flux and the electric field outside the confined process region.These measurements confirmed that the electric field and the ion flux inregions outside of the process region were substantially lower than inthe prior art.

As can be seen from the foregoing, the present invention offers numerousadvantages over the prior art. Different embodiments or implementationsmay have one or more of the following advantages. One advantage of thepresent invention includes confining a plasma to a process region of theprocess chamber, while permitting by-product gas from processing to passthrough. Another advantage of the present invention includes minimizingand/or eliminating unwanted plasma formations in the regions outside ofthe process region of the process chamber. Accordingly, the plasma canbe controlled to a specific volume and a specific location inside theprocess chamber, ensuring more efficient coupling of energy, enhancingplasma uniformity, and increasing plasma density, all of which lead tobetter processing uniformity and high yields on processed substrates.

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and equivalents, whichfall within the scope of this invention. It should also be noted thatthere are many alternative ways of implementing the methods andapparatuses of the present invention. For example, although theconfining assembly has been described and shown in terms of acapacitively coupled plasma reactor for processing substrates, it shouldbe noted that other plasma systems could apply the techniques andmethods of the confining assembly. For example, it is contemplated thatthe confining assembly could be used in inductively coupled or microwaveplasma reactors. It is therefore intended that the following appendedclaims be interpreted as including all such alterations, permutations,and equivalents as fall within the true spirit and scope of the presentinvention.

1. A plasma confining assembly for minimizing unwanted plasma formationsin regions outside of a process region in a process chamber, comprising:a first confining element positioned proximate the periphery of theprocess region, and including a conductive surface that is electricallygrounded and exposed to said process region; and a second confiningelement positioned proximate the periphery of the process region, andincluding an exposed insulating surface, which is configured to at leastpartially cover a non-exposed conductive core that is electricallygrounded, the second confining element being spaced apart from the firstconfining element such that one of the confining elements is disposed inan upper portion of the process chamber and the other confining elementis disposed in a lower portion of the process chamber, wherein the firstconfining element and the second confining element substantially reducesthe effects of plasma forming components that pass therebetween.
 2. Theplasma confining assembly as recited in claim 1 further including athird confining element formed from an insulating material and disposedbetween the first confining element and the second confining element,and proximate the periphery of the process region, the third confinementelement being arranged to physically contain a plasma inside the processregion and to substantially reduce the effects of plasma formingcomponents that pass between the first confining element and the secondconfining element.
 3. The plasma confining assembly as recited in claim2 wherein the third confining element is a ring that surrounds at leasta portion of the process region, the third confining element beingconfigured to permit by-product gas from the processing to pass throughwhile substantially confining the plasma inside the process region. 4.The plasma confining assembly as recited in claim 1 wherein the plasmaforming components are charged particles or electric fields.
 5. Theplasma confining assembly as recited in claim 4 wherein the firstconfining element and the second confining element are arranged todirect charged particles to the exposed conductive surface and sinkcharged particles therethrough to ground so as to reduce the density ofcharged particles in regions outside of the process region.
 6. Theplasma confining assembly as recited in claim 4 wherein the firstconfining element and the second confining element are arranged toattract electric fields to the grounded conductive surface and thegrounded conductive core, respectively, so as to reduce the electricalfield strength in regions outside of the process region.
 7. The plasmaconfining assembly as recited in claim 1 wherein the first confiningelement is disposed in an upper portion of the process chamber, andwherein the second confining element is disposed in a lower portion ofthe process chamber.
 8. The plasma confining assembly as recited inclaim 1 wherein the first confining element is disposed in a lowerportion of the process chamber, and wherein the second confining elementis disposed in an upper portion of the process chamber.
 9. The plasmaconfining assembly as recited in claim 1 wherein the non-exposedconductive core is formed from aluminum and wherein the exposedinsulating surface is formed from anodized aluminum.
 10. The plasmaconfining assembly as recited in claim 1 wherein the conductive surfaceof the first confining element is formed from an electrically conductingmaterial that is either substantially resistant to etching by a plasmapresent within the chamber during the processing or contributessubstantially no metal contamination.
 11. The plasma confining assemblyas recited in claim 1 wherein the exposed conductive surface faces theexposed insulating surface such that the exposed insulating surface isdisposed between the exposed conductive surface and the non exposedconductive core.
 12. The plasma confining assembly as recited in claim 1wherein the insulating surface prevents electrons or negative ions frombecoming trapped between the exposed conductive surface and the nonexposed conductive core.
 13. The plasma confining assembly as recited inclaim 1 wherein the exposed conductive surface that is grounded and theexposed insulating surface that covers a non-exposed conductive corethat is electrically grounded cooperate to form a DC potentialtherebetween when an RF voltage is supplied to the process chamber, theDC potential guiding charged particles to the exposed conductive surfacethat is grounded, the exposed conductive surface that is groundedsinking the guided charged particles therethrough to ground so as toreduce the density of charged particles in regions outside of theprocess region.
 14. A plasma confining assembly for minimizing unwantedplasma formations in regions outside of a process region in a processchamber, comprising: a first confining element positioned proximate theperiphery of the process region, and including a conductive surface thatis electrically grounded and exposed to said process region; and asecond confining element positioned proximate the periphery of theprocess region, and including an exposed insulating surface, which isconfigured to cover a non-exposed conductive core that is electricallygrounded, the second confining element being spaced apart from the firstconfining element such that one of the confining elements is disposed inan upper portion of the process chamber and the other confining elementis disposed in a lower portion of the process chamber, wherein the firstconfining element and the second confining element substantially reducesthe effects of plasma forming components that pass therebetween, whereinthe first confining element is disposed in an upper portion of theprocess chamber, and the second confining element is disposed in a lowerportion of the process chamber, and wherein the first confining elementis a ring that surrounds an upper electrode, and the second confiningelement is a ring that surrounds a bottom electrode, the upper andbottom electrode being arranged for producing an electric field thathelps to ignite and sustain a plasma.
 15. A plasma confining assemblyfor minimizing unwanted plasma formations in regions outside of aprocess region in a process chamber, comprising: a first confiningelement positioned proximate the periphery of the process region, andincluding a conductive surface that is electrically grounded and exposedto said process region; and a second confining element positionedproximate the periphery of the process region, and including an exposedinsulating surface, which is configured to cover a non-exposedconductive core that is electrically grounded, the second confiningelement being spaced apart from the first confining element such thatone of the confining elements is disposed in an upper portion of theprocess chamber and the other confining element is disposed in a lowerportion of the process chamber, wherein the first confining element andthe second confining element substantially reduces the effects of plasmaforming components that pass therebetween, wherein the first confiningelement is disposed in a lower portion of the process chamber, and thesecond confining element is disposed in an upper portion of the processchamber, and wherein the first confining element is a ring thatsurrounds a bottom electrode, and the second confining element is a ringthat surrounds an upper electrode, the upper and bottom electrode beingarranged for producing an electric field that helps to ignite andsustain a plasma.
 16. A plasma confining assembly for minimizingunwanted plasma formations in regions outside of a process region in aprocess chamber, comprising: a first confining element positioned at aboundary between the process region where a plasma is ignited andsustained for processing a work piece and the regions outside of theprocess region where the plasma is not desired, the first confiningelement including a conductive member that is exposed within the processchamber to said process region, the conductive member being electricallygrounded; and a second confining element positioned at the boundarybetween the process region where the plasma is ignited and sustained forprocessing and the regions outside of the process region where theplasma is not desired, the second confining element including aninsulating portion that is exposed within the process chamber, and aconductive portion that is covered by the insulating portion so as tokeep the conductive portion from being exposed inside the processchamber, the conductive member being electrically grounded, the secondconfining element being spaced apart from the first confining element soas to form an open area therebetween that permits by-product gases topass therethrough from the process region to the regions outside of theprocess region while substantially preventing charged particles orelectric fields from passing therethrough from the process region to theregions outside of the process region, wherein the first confiningelement is formed as a first ring configured to surround a firstelectrode, and wherein the second confining element is formed as asecond ring configured to surround a second electrode that is spacedapart and parallel to the first electrode, the first and secondelectrodes defining the process region therebetween, the first andsecond electrodes being configured for generating an electric field thatis sufficiently strong to both ignite and sustain the plasma in theprocess region of the process chamber.
 17. The plasma confining assemblyas recited in claim 16 further including a pressure control ring formedfrom a dielectric medium and disposed between the first and secondrings, the pressure control ring being configured for physicallyconfining a plasma within the process region, while permitting thepassage of process gases to pass therethrough.
 18. The plasma confiningassembly as recited in claim 16 wherein the exposed insulating surfaceis configured to be level with a top surface of the second electrode.19. The plasma confining assembly as recited in claim 16 wherein thefirst ring is configured to be disposed between the first electrode anda chamber wall of the process chamber, and wherein the second ring isconfigured to be disposed between the second electrode and the chamberwall of the process chamber.
 20. The plasma confining assembly asrecited in claim 19 wherein the first ring is spaced apart laterallyfrom the chamber wall thus leaving an open area between the first ringand the chamber wall.
 21. The plasma confining assembly as recited inclaim 16 wherein the first ring includes an inner ring and an outerring, wherein the inner ring is formed from a dielectric medium and isconfigured to be disposed between the first electrode and the outerring, and wherein the outer ring includes the conductive member of thefirst ring.
 22. The plasma confining assembly as recited in claim 16wherein the second ring includes an inner ring and an outer ring,wherein the inner ring is formed from a dielectric medium and isconfigured to be disposed between the second electrode and the outerring, and wherein the outer ring includes the conductive portion and theinsulating portion.
 23. The plasma confining assembly as recited inclaim 16 wherein the conductive element is a portion of the processchamber.
 24. The plasma confining assembly as recited in claim 16wherein the first ring and the second ring are configured to extend in aradial direction relative to an axis of the process chamber, and whereinan outer edge of the first ring extends further than an outer edge ofthe second ring.
 25. The plasma confining assembly as recited in claim16 wherein the first and second confining elements are configured to belocated between the process region and an exhaust port.
 26. The plasmaconfining assembly as recited in claim 16 wherein the exposed conductivemember of the first confining element and the exposed insulating portionof the second confining element each include surfaces that aresubstantially parallel to one another and that are perpendicular to theboundary between the process region where a plasma is ignited andsustained for processing a work piece and the regions outside of theprocess region where the plasma is not desired.
 27. A plasma confiningassembly for minimizing unwanted plasma formations in regions outside ofa process region in a process chamber, comprising: a first confiningelement including a conductive surface that is electrically grounded andexposed to said process region, the exposed conductive surface that iselectrically grounded being configured to sink charged particlestherethrough to ground so as to reduce the density of charged particlesin regions outside of the process region, the exposed conductive surfacethat is electrically grounded also being configured to attract electricfields so as to reduce the electrical field strength in regions outsideof the process region; and a second confining element including anexposed insulating surface, the exposed insulating surface covering anon-exposed conductive core that is electrically grounded, theinsulating surface preventing charged particles from sinking into thenon-exposed conductive core that is electrically grounded, thenon-exposed conductive core that is electrically grounded beingconfigured to attract electric fields so as to reduce the electricalfield strength in regions outside of the process region.